Trump hailed it as "one of the most important, and largest, Trade Deals in US and World History" on Twitter shortly after the signing ceremony.
"Revamping the new trade agreement was aimed to preserve the view of an integrated North America with the firm belief that together we are stronger and more competitive", he said in remarks translated from Spanish.
Meanwhile, a number of lawmakers and business groups are trying to push for the removal of US tariffs on steel and aluminum imports from Canada and Mexico, which have inflicted harm upon American manufacturers and consumers, raising costs for a wide array of industries.
The United States, Mexico, and Canada reached a tentative agreement to strike a new North American trade deal, and President Donald Trump has given Congress six months to approve the measure.
Trump, joined by Canadian Prime Minister Justine Trudeau and outgoing Mexican President Enrique Peña Nieto, signed the United States-Mexico-Canada Agreement during a ceremony before the start of this year's G-20 summit in Buenos Aires, Argentina.
Pelosi, who is expected to take the role of House Speaker in January, has repeatedly expressed deep concerns over the deal's enforceability.
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In a blog entry posted shortly after the agreement was signed Friday, Cato Institute trade analyst Simon Lester appeared to anticipate Trump's move - although he acknowledged it would have made a lot more sense if the Republicans still had control of Congress. "A great deal." Trump said Saturday, during a press conference on Air Force One.
A spokesman for Foreign Affairs Minister Chrystia Freeland said Trump's comments are part of the American ratification process.
"I think they're betting on Congress not letting it all go to hell", she said.
"Make no mistake, we will stand up for our workers and fight for their families and their communities", he said. "While we'll have to watch and ensure we get through this next stage, we have a high level of confidence that's achievable".
"The ceremonies, the signing - the president's very happy with all of that".